Apparatus for shortening of electron pulses emitted from an electron gun



July 8 1969I D FROM J. SCSI-FER ETAL APPARATUS FOR SHORTENING OF ELECTRON PULSES EMITTE Filed Sept. 2. 1966 AN ELECTRON GUN Sheet INVENT'oRs:

July 8 1969 J. soFFl-:R ETAL Y3,454,818 APPARATUS FOR SHORTENING OF ELECTRON PULSES EMITTED FROM AN ELECTRON GUN Filed sept. 2, 1966 sheet Z of :s

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Jacques SOI-'FER July 8 1969 J. SOF FER ET AL APPARATUS FOR SHORTENING oF ELECTRON PULsEs EMITTED FROM AN ELECTRON GUN 3 afs Sheet Filed Sept. 2, 1966 INVENToRs Jcl/es 60FFR v/ een Fu/ HANG/N BY 9;.

ATTORN United States Patent O U.S. Cl. 315-5.41 14 Claims ABSTRACT OF THE DISCLOSURE Apparatus providing electron pulses of short duration wherein a pulse duration T is converted into a series of individual electron bursts, each series having a duration T, and subsequently re-grouping the individual bursts of each series into a short pulse of duration T.

This invention relates in general to electron discharge devices, and more particularly to apparatus for generating electron pulses of extremely short duration emitted from an electron gun for injection into a device Where such pulses are extremely desirable such as an accelerator for example.

It is desirable, especially in the disclosed example, to reduce the pulse length as near as possible to the theoretical limit imposed by the non-linearities of the phenomena involved in the technique as well as by the space charge. This limit is about 1.5 nanoseconds, and therefore it is desirable to achieve pulse lengths of the order of 2 to 3 nanoseconds.

A pulse length of this order of magnitude cannot be attained directly by applying a pulse shaped control onto the control electrode of the electron gun since the time constant of the capacitive grid-cathode circuit limits the minimum practicable pulse length to the order of 6 nanoseconds. Moreover, if high intensities are to be obtained without applying high voltages to the gun electrodes, it is preferred to operate above this limit and not to reduce the pulse length below about 10` to l2 nanoseconds.

However, for injection into a device such as an accelerator it is necessary to reduce the length of the pulses emitted from an electron gun by the ratio 1:4 to 1:6; and it is an object of the present invention to provide an improved apparatus by which this may be achieved.

The present invention consists of an apparatus for shortening electron pulses emitted from an electron gun controlled by a signal having a repetition frequency f, including a buncher device positioned on the path of the electron stream for converting each pulse generated by the electron gun into a series of electron bursts, and further includes a debuncher device positioned adjacent the buncher device for subsequently causing the electron bursts formed by the buncher to interact with a high frequency wave in such a manner that some bursts of each series drift with respect to the waveto a greater extent than other bursts of the same series, whereby the bursts are debunched and regrouped so as to form a nal pulse having a duration shorter than the initial one generated by the electron gun.

It is therefore a principal object of the present invention to provide an electron beam generating arrangement for producing pulses of electrons of extremely short duration.

It is another object of the present invention to provide an electron beam generating arrangement for use in connection with accelerators for producing a high energy electron stream consisting of bunches of short length.

It is a further object of the present invention to provide an electron beam generating arrangement fo use in connection with accelerators for producing pulses of electrons of extremely short duration and high energy without the application to the beam generator of control signals of high voltage.

These and other objects, features and advantages will become more apparent from the following detailed description of the invention when taken with the accompanying drawings, which disclose various exemplary embodiments of the invention, and wherein:

FIGURE 1 represents schematically one form of apparatus according to the present invention;

FIGURE 2 is an explanatory diagram relating to the apparatus of FIG. l;

FIGURE 3 shows the apparatus of FIGURE 1 used for electron injection in a linear accelerator;

FIGURE 4 represents schematically an alternative form of apparatus according to the present invention; and

FIGURE 5 is an explanatory diagram relating to the apparatus of FIG. 4.

In carrying the invention into effect according to one convenient mode, by way of example, FIGURE l schematically shows apparatus which includes an electron gun 1 emitting a beam of electrons which propagate along the dotted axis 2. The gun 1 is controlled by a source 3 of signals having a frequency f, for example j=3333 cycles/ sec. Under this control the electrons are emitted in the form of pulses with a repetition frequency of 3333 c./sec. and spaced consequently by 300 microseconds. It will be assumed that without excessive expense in the energy supply apparatus it is possible to obtain pulses of a high intensity and a duration of T=12 nanoseconds.

There is disposed on the path 2 of the electrons a bunching device 4 adapted to produce a series of electron bursts, which device 4 is formed by a chain of p resonator cavities uniformly spaced apart and excited at a frequency f1 by a signal source 5. Thereafter, there is provided guide portion 6 excited at a frequency f2 by a signal source 7 having an energy that is not suicient to accelerate the electrons by means of the phenomenon of trapping the electron bursts by a xed phase of the wave. The source 5 is in the form of a mixer in which the frequency f2, for example f2=l200 mc./sec., beats with the frequency mf obtained from a frequency multiplier 8 that multiplies the frequency f by a factor m, which may take the form of, for example, a cascade of eight triplers producing a multiplying factor of m=38=6561, whereby From the mixer 5 there is derived the frequency sum -1-21.65=l22l.65 mc./sec. which is the frequency f1 feeding the buncher 4. The frequency f2 is consequently slightly lower than f1. An adjustable phase shifter 9 is interposed between the elements S and 5 for purposes to be more fully described hereinafter.

This apparatus of the invention, as illustrated in FIG- URE l, operates in the following manner:

In FIGURE 2 the diagram a represents, as a function of time, a pulse of duration T, emitted from the gun 1 and entering the buncher device 4 (point a in FIGURE l). In the buncher 4, the electrons contained in the pulse interact with the wave of frequency f1 whose curve is represented by diagram b. Owing to the well known phenomenon wherein electrons are trapped by a xed phase, represented by the points A and the like, of high frequency wave energy through which they pass, the electrons ar-e bunched or grouped in a series of bursts p represented in diagram c. This series occupies substantially the same total duration T as the initial pulse, and the width s of each burst corresponds approximately to 20 of the phase variaaimais E tion of the wave f1. This series of bursts enters at c (FIG' URE l) in the guide 6.

In this guide there propagates a wave of frequency f2 whose curve is represented in diagram d. By adjusting the phase shifter 9 to selectively adjust the phase of the signal mIf with respect to f2 it can be arranged that the beat node of the frequencies f2 and mf coincides with the instant at which the middle burst pm of the series enters into the guide. Under these conditions both of the waves f1 and f2 are in phase at a point thereof corresponding to the middle of the initial pulse represented in diagram a.

It may then be seen in the diagrams c and d that the first burst p1 enters the guide 6 when the phase of wave f2 is B, lagging by a phase angle a1 behind the wave peak, whereas the last burst pn enters the guide 6 when the phase of wave f2 is C, leading by a phase angle a2 ahead of the wave peak. Between the input phases of the first and last bursts there is a phase shift equal to the sum of the absolute values of al and a2. This phase shift is equal to ot=21rmfT For mf=21.65 mc./sec. and T=12 nsec., 294

Owing to the adjustment of the phase shifter 9 and to the fact that the frequency my is a multiple of the frequency f, the same conditions are maintained on all consecutiVe pulses, i.e., the phase shift between corresponding bursts on either side of the center burst is equal to the absolute Vahle of the sum of the individual input phases of the respective burst.

Consider now the fact that the wave f2 propagates without delay, that is at the speed of light, while the electrons have 'been subjected to relatively little acceleration within the buncher 4 and consequently propagate at a considerably lower speed. Moreover, the bursts cannot be trapped by the wave because the energy of the wave is insufficient to initiate this phenomenon. The bursts will therefore lag with respect to the wave.

The bursts, such as p1, which lag behind the wave and the bursts, such as pn, which lead the wave, behave in different manners. The two groups are at the beginning in the accelerating field corresponding to the positive portions of curve f2. But the burst p1 attains the negative portion of the curve f2, corresponding to the decelerating field, after a drift along the short Wave portion BD, corresponding to the phase angle l, whereas the burst pn attains the negative portion of the curve, that is, the decelerating field, after a much longer drift along the wave portion CE, corresponding to the phase angle ,B2 which is much greater than the angle l, as seen in the diagram. Consequently, the burst pn remains in the accelerating field longer than the burst p1. 'Ihe bursts such as p,n therefore acquire in the guide a mean velocity which is higher than that of the bursts such as p1, and the former catch up with the latter during their propagation. At the same time the bunching of the bursts is destroyed and the electrons join again to form a continuous pulse at the output e of the guide (FIGURE 1) which acts as a debuncher. This pulse, represented in the dia-gram e of FIGURE 2, has a duration T shorter than the initial duration T. The relative reduction of the duration corresponds to the ratio s/a, that is, in the example given 20/94=l:4.7. The initial duration of l2 nanoseconds is thus reduced to about 2.5 nanoseconds.

FIGURE 3, in which the same reference numerals as in FIGURE 1 designate the corresponding elements, represent-s schematically an application of the apparatus of FIGURE 1 for injecting electrons into a linear accelerator. The shortened pulses issued from guide 6 enter a preliminary conventional buncher 10 wherein the pulse is bunched into groups or bursts. Then the bursts enter the accelerating section 11, constructed in conventional manner for accelerating the electrons to a speed nearly equal to the speed of light and giving them high energy. If necessary, the accelerated bursts issued from section 11 may be -grouped into a shortened group with the aid of a regrouper device 12 such as the one described in our copending U.S. patent application Ser. No. 438,656, -filed Mar. l0, 1965. The electrons issued from the regrouper 12 or directly from accelerator 11 are directed toward a device for their use, not shown.

The preliminary buncher 10 and the accelerator 11 may be supplied with high frequency at any suitable frequency f3 and at a high power level. Moreover, in order to reduce the number of sources employed, it is advantageous to take f3=f2 and utilize the source 7 of frequency f2 both for supplying full power to the elements 101 and 11 and reduced power, through an interposed attenuator 13, to the guide 6 and the mixer 5.

In FIGURE 4 the block 1 represents, as in FIGURE 1, an electron gun emitting electrons that propagate along the dotted axis 2. The electrons are emitted in pulses having a length of the order of l2 nanoseconds at a repetition frequency f of the order of a few kilocycles/sec. In the path 2 of the electrons there is disposed a buncher 14 excited at a frequency f4, for example of the order of 3000 rnc/sec., supplied by a source 15 which is deeply a-mplitude modulated at a frequency F, for example, of the order of 20 mc./sec. (a multiple of f), supplied by a source 16 which rnay be a multiplier of the frequency f, locked to the source 3 of the latter, thereby maintaining the phase synchronism. The length of the buncher 14 is chosen to be of the order of vg/2F, Where vg is the group velocity of the modulated wave during the propagation thereof in the buncher. With )1:3000 mc./sec. the buncher may be constructed in such a manner that and then, for F=20 mc./sec., the length of the buncher is of the order of 20 cm.

The buncher 14 extends into an accelerating guide 17, in which the modulated wave propagates with a constant (phase velocity nearly equal to that of light. At the output of this guide the electrons are injected, for example, into a high energy accelerator 118, analogous to block 11 of FIGURE 3, and excited by a HF source 19.

This apparatus in accordance with the invention, as illustrated in FIGURE 4, operates as follows:

Suppose that the modulating magnitude supplied by source 15 is such that the energy exciting the buncher varies sinusoidally at the frequency f4, amplitude modulated at frequency F. This energy produces a field which varies sinusoidally in the space between zero and a peak value. This sinusoidal distribution of period vg/F propagates in the buncher with a group velocity vg. If the length of the buncher is equal to half a period of the spatial distribution, the field along the buncher Varies periodically in time between two spatial distributions, one of which is represented by the curve A and the other by the curve B in FIGURE 5, which provides a field which is a function of the axial coordinate of the buncher. It is thus seen that the electrons which enter the buncher during the the field has the distribution A meet from the beginning a weak field so that they are accelerated relatively little and leave the buncher with a relatively low mean velocity. On the other hand, the electrons which enter the buncher during the distribution B encounter during their entire passage through the buncher a relatively intense field which gives them a strong acceleration and a relatively high mean velocity. These electrons group themselves into bursts near the peak of the wave (phase 1r), while the preceding ones group themselves into bursts near the phase vr/ 2.

With correctly adjusted phases, the electrons issuing from the first half of the initial pulse arrive during the field distribution A and those of the second half arrive during the field distribution B.

When the two kinds of bursts enter the accelerator guide 17, they behave in different manners. Those which arrive with the higher mean velocity at the phase 1r (corresponding tQ the second half of the initial pulse) are trapped by the wave that propagates with a constant phase velocity or drift relatively slowly behind the wave. Those which arrive with the lower mean velocity at the phase 1r/2 (corresponding to the first half of the initial pulse), are

, not trapped by the wave and drift rapidly behind. The

mean velocity of the former continues to be higher than that of the latter, the former catch the latter ones thereby producing a degrouping or debunching, and at the output of guide 17 which acts as a debuncher there appears a shortened pulse of the form shown in diagram e of FIGURE 2.

These conditions of operation are repeated for each pulse emitted from the electron gun due to the multiple factor which ties together the frequencies F and f and the phase synchronism.

The modulation of source need not be carried out sinusoidally; it suffices to modulate the duration or position of the pulses such that the output power of the source 1S varies as an arc of a sinusoid just for the duration of the pulse issuing from the electron gun.

While lwe have shown and described several embodiments according to the present invention, it is understood that the same is not limited thereto but is susceptible of numerous changes and modifications as known to a person skilled in the art. Therefore, we do not wish to be limited to the details shown and described herein, but intend to cover all such changes and modifications as are encompassed by the scope of the appended claims.

We claim:

1. An apparatus for shortening the length of electron pulses of duration T emitted from an electron gun with a repetition frequency f, comprising buncher means positioned along the path of the electrons for converting each pulse into a series of individual electron bursts, each series having a duration T, and debuncher means positioned along said path subsequent to said buncher means in the direction of said electron emission for grouping the individual electron bursts in each series including means for effecting interaction between the electron bursts and a high frequency wave of frequency f2 in such a manner that some bursts of each series drift with respect to the wave to a greater extent than other bursts of the same series, whereby the bursts are debunched and regrouped so as to form a final pulse having a duration T' shorter than the initial duration T.

2. Apparatus as claimed in claim 1, wherein the electrons interact in the buncher device with a high frequency wave of a frequency f1, which is slightly higher than the frequency f2 of the wave which propagates in the debuncher device.

3. Apparatus as claimed in claim 2, wherein the difference between the frequencies f1 and f2 is a frequency mf, which is an integral multiple of the repetition frequency.

4. Apparatus as claimed in claim 2, further including mixing means for producing the frequency f1, as the sum beat frequency obtained by mixing the frequencies mf and f2.

5. Apparatus as claimed in claim 4, further including means connected to said mixing means for adjusting the phase of the wave of frequency mf to provide a beat node of the frequencies f1 and f2 at the center of each series of electron bursts.

6. Apparatus as claimed in claim l1 wherein the buncher device causes the electrons to interact with a high frequency wave amplitude modulated at a frequency F, while the debuncher device is an accelerator guide in which the amplitude modulated wave propagates with a constant phase velocity equal to the velocity of light.

7. Apparatus as claimed in claim 6, wherein the frequency F is a multiple of f, and wherein the phases of the two frequencies are maintained in synchronism.

8. Apparatus as claimed in claim 7 lwherein the length of the buncher device is substantially equal to vg/ZF, vg being the group velocity of the modulated wave.

9. Apparatus for producing pulses of electrons having an extremely short duration comprising electron beam generating means for generating a stream of electrons along a given path, modulating means applying a control signal of frequency f to said beam generating means for modulating said stream of electrons into a plurality of pulses of duration T, buncher means positioned along said path for converting each pulse of electrons into a series of individual electron bursts, each series having a duration T, and debuncher means positioned along said path subsequent to said buncher means lfor causing some bursts of each series to drift with respect to other bursts of said series toward a common point of the series, whereby the bursts are regrouped to form a final pulse having a duration T shorter than duration T.

10. The combination defined in claim 9 wherein said buncher means includes signal means for effecting interaction between said pulses of duration T and a high frequency wave of frequency f1, equal to mf plus a frequency f2, where m is an integer and mf is very small in comparison with f2.

11. The combination defined in claim 10 wherein said signal means includes multiplier means connected to said modulating means for generating a signal having a frequency mf, generator means for generating a signal having a frequency f2, and mixing means for adding the signal outputs from said multiplier means and said generator means for producing a signal having a frequency f1, said buncher device further including a chain of resonator cavities through which said electrons pass, which cavities are excited by said signal of frequency f1.

12. The combination defined in claim 11 wherein said debuncher means includes a waveguide excited by said signal of frequency f2 and within which said electrons interact with a wave of frequency f2.

13. The combination defined in claim 12 further including phase shifting means connected between said multiplier means and said mixing means for providing a phase shift between the phases of the first and last bursts of a series entering said waveguide with respect to the wave of frequency f2.

14. The combination defined in claim 12 wherein said phase shift is equal to 21rmf1`.

References Cited UNITED STATES PATENTS 3,070,726 12/1962 Mallory 3l5-5.42 3,333,142 7/1967 Takeda et al. 328-233 HERMAN KARL SAALBACH, Primary Examiner. P. L. GENSLER, Assistant Examiner.

U.S. C1. X.R. 37-106; 328--233 

